In foundry installations, molten metal may be discharged from a bottom discharge holding reservoir into a mold. The flow control of molten metal from the reservoir to the mold is extremely important for the successful molding of metal parts. Accordingly, a stopper rod control mechanism is used to control the flow of molten metal through the reservoir's nozzle into the mold. The stopper rod mechanism thereby insures that the sprue cup of the mold is properly filled at all times during molding so that the molten metal propagates through the gating system of the mold and then into mold cavities.
Two problems inherently arise with the use of the stopper rod control mechanism. The first problem is associated with the flow rate of metal through the nozzle; the second, with the complete stoppage of metal flow through the nozzle.
The solution to the first problem requires that the stopper rod be quickly reciprocated in the vertical direction and quickly stopped to insure a proper rate of flow of material through the nozzle. The speed and precision of this vertical movement is important to insure proper flow rate so that the sprue cup of the mold is always properly filled.
The solution to the second problem requires that the rod be properly seated within the nozzle thereby insuring complete closure of the nozzle. This solution requires consideration of several factors. The rod must be movable in relation to the nozzle so that the rod can be properly aligned with the nozzle. Slag formation upon the nozzle is prevalent in these molding operations, so it is necessary to be able to vary the seating force (prestress) of the rod so that slag can be crushed off the nozzle. However, this force cannot be so great that it will deform the rod and nozzle.
In response to these problems, various manufacturers have attempted to solve them by primarily pneumatic means. For example, a mechanism manufactured by Seaton includes an air cylinder to operate the initial vertical movement of the stopper and the stroke of the stopper rod is adjusted with a ball screw and motor mechanism.
A mechanism manufactured by Brown Boveri actuates the vertical movement of the stopper rod with an air operated cylinder which is controlled by solenoids. Additionally this mechanism has the following disadvantages: no flexibility to allow alignment between a mismatched nozzle and rod; the stopper rod is not easily replaced; the position and velocity control of the rod is not possible; and the seating force of the rod into the nozzle is not a variable.
a mechanism manufactured by ASEA uses an air cylinder to actuate vertically reciprocal linear movement of the stopper rod. The stroke of the stopper rod is adjusted by a second air cylinder.
A mechanism manufactured by George Fisher includes a pneumatic diaphragm which operates a parallelogram mechanism that moves the stopper rod up and down. This mechanism includes a feature which allows the stopper rod to be twisted.
The above described mechanisms have the following disadvantages which adversely affect the use of such mechanisms and the process of filling mold:
1. low speed;
2. excessive and uncontrollable seating force of the rod in the nozzle (prestress);
3. arc motion instead of strict vertical linear motion of the rod;
4. no provision for accurately aligning the stopper rod and the nozzle; and
5. a laborious stopper rod replacement procedure.